1. Field of the Invention
This invention pertains to the field of metal-air electrochemical energy converters, as well as solid fuel cells. More particularly, it pertains to a fuel cell that uses carbon directly as a "fuel" anode in an electrochemical cell with atmospheric air as the oxidizer and to design and operational modifications thereof that allow the fuel cell to be commercially viable.
2. Description of the Prior Art
The element carbon (C) has an affinity for combining chemically with a large number of other chemical elements under a variety of conditions. In this instance, carbon has a chemical affinity for oxygen in the air and when brought into contact at a proper temperature, carbon enters into chemical combination with the oxygen in an exothermic reaction known as "combustion". However, as discovered by W. W. Jacques in 1896, carbon can be chemically combined with oxygen in the air, not directly as in combustion, but through an intervening electrolyte to produce predominantly electrical energy instead of heat.
This process of converting the potential energy of carbon into electrical energy has great potential for the transition to a sustainable energy infrastructure for the world's peoples. The world's oil and gas resources, as well as coal reserves, are being used inefficiently and are causing increasing pollution at an accelerating rate when used to provide heat, electricity and run industry. Although new reserves and resources are constantly being discovered, the cost of extracting them is also rising. The damage to the environment from mining, drilling and leaking pipelines, as well as inefficient and incomplete combustion, has caused pollution of the air and water, and destruction of wetlands, fishing grounds and forests.
The combination of the carbon-air fuel cell and using renewable biomass as a source of the carbon fuel (charcoal) eliminates all of these problems through the fuel cell's much higher efficiency and lower operating temperature. Also, using the solar biomass fuel cycle, there is no net increase in CO.sub.2 and no emission of toxic pollutants such as NOx, SOx, CO or volatile organic compounds (VOC's). By undertaking an increase in production of certain flora, on biomass plantations, using marginal and degraded lands not in conflict with conventional agriculture, carbon dioxide can be removed from the atmosphere and converted to biomass, and thence to carbon rods, sticks, sheets, slabs, chunks and other forms such that they can become the anodes for carbon-air fuel cells. This will provide electrical power for stationary, portable and mobile applications, replacing the common internal combustion engine, diesel engine and gas turbine, giving greatly improved performance in electric vehicles over conventional lead-acid batteries.
The use of carbon directly as a "fuel" anode in an electrochemical cell was first practiced by Jacques. In his 1896 patent, he bubbled air upward through a molten salt electrolyte of sodium hydrate (caustic soda and sodium hydroxide), at a temperature between 400.degree. to 500.degree. C., to cause an ebullition of oxygen-saturated electrolyte that was then placed into contact with a carbon anode. The carbon was oxidized to form carbonic acid gas (carbon dioxide) and an electrochemical potential was developed between the carbon rod and the iron containment pot, that resulted in an electric current flowing through an external load connected between them. The container (from Jacques) was made from the inert metal, platinum, or Norway iron (ultra low carbon steel).
Jacques added magnesium oxide, a basic oxide, to the electrolyte, in a partially successful attempt to suppress carbonate formation. The tendency of the electrolyte was to react with the byproduct CO.sub.2 to form sodium carbonate. If the carbonate builds up beyond 30% by weight of the electrolyte, it will begin to adversely affect the cell's operation. With the addition of magnesium oxide, Jacques was able to increase the operational life of his cells from several days to several months before exceeding the 30% limit. At the end of the operating time the electrolyte had to be removed from the cells and regenerated to remove the carbonate.
Anbar and Weaver's patents were different from Jacques in that they used a molten carbonate electrolyte. Their two patents included a lead anolyte, but published research results indicated they dropped the lead anolyte and reverted to the Jacques geometry in its slab form. Their cell was not commercially viable for the following reasons: the operating temperature was between 650.degree. and 750.degree. C.--this caused severe corrosion problems (the same problems suffered by hydrogen fueled molten carbonate fuel cells); the electrolyte conductivity is low--resulting in poor current density and power density; the electrolyte would decompose from carbonate into oxide-evolving CO.sub.2. Only one of these problems has been solved technically, and that is the electrolyte decomposition. By introducing a partial pressure of CO.sub.2 into the cathode air feed, the decomposition is overwhelmed by the formation of more carbonate. In a practical system, this would require recovering 2/3 of the CO.sub.2 in the exhaust, and reinjecting it into the fresh air feed. The only technically feasible method for doing this at present, pressure swing absorption, is not commercially viable, and is too bulky for portable or mobile applications.
Over the years there have been improvements in electrolyte compositions, matrix materials, fabrication techniques, as well as in anode and cathode materials. Yet, no one has applied these improvements to the carbon-air fuel cell and the result has been that it has not yet become a viable alternative to the internal combustion engine, diesel engine, gas turbine or lead-acid battery.